Patent GB-729677-A (GB)
Title: Improvements in or relating to a process for obtaining mannitol from plant-extracts
Filed: 1953-01-19
Abstract: Mannitol is extracted from plant extracts, e.g. seaweed extract, by treating the extract in an alkaline medium with a copper salt to convert the mannitol into a complex copper-mannitol compound and decomposing this with an acid, characterized in that the complex copper-mannitol compound is separated and decomposed with an organic acid containing at least six carbon atoms, whereby two phases are obtained, one consisting of a solution of a copper salt in the organic acid, whilst the mannitol is obtained from the other phase. The complex copper-mannitol compound is preferably decomposed with the organic acid in the presence of water and the mannitol recovered from the aqueous phase.  The process may also be carried out by adding a water-immiscible organic solvent to the complex copper-mannitol compound and the organic acid in the presence or absence of water, both the copper salt and the free organic acid entering into solution in such solvent.  Generally, any organic acid containing at least six carbon atoms, the copper salt of which is soluble in the acid itself and/or in the usual water-immiscible organic solvents, such as benzene, petroleum-benzene or chloroform, may be used.  Specified organic acids are saturated fatty acids; commercial fatty acid mixtures usually comprising unsaturated fatty acids, e.g. a mixture of linoleic acid and oleic acid containing a small amount of saturated fatty acid; and naphthenic acids.  The decomposition of the complex with the acid is preferably effected at 80-90 DEG  C. The copper salt may be regenerated from the organic phase by washing the latter with an acid, e.g. sulphuric acid, whereby the copper passes into the sulphuric or other acid layer in the form of the relevant copper salt and the organic acid used is liberated and remains in the organic phase.  It is advisable to remove the iodine content of the seaweed extract before use, e.g. by treating the extract with an oxidizing agent and separating the iodine formed, e.g. by distillation, or by treating the extract in an acid medium with a copper salt with the addition of a sulphite or free sulphurous acid whereby a cuproiodide, which may be recovered by filtration or centrifugation, separates.  Examples are given in which a seaweed extract, e.g. an extract obtained by extraction of seaweed with dilute hydrochloric acid, is precipitated as a complex compound in an alkaline medium with copper sulphate, the subsequent treatment of the precipitate for the recovery of mannitol being carried out by suspending the complex in water and adding an excess of either (1) the mixed fatty acids of arachis oil at about 50 DEG  C; (2) ethyl-hexoic acid at about 50 DEG  C.; (3) technical stearic acid at about 90 DEG  C.; (4) a mixture of equal parts of white spirit and a commercial mixture of naphthenic acids; or (5) a mixture of distilled tall oil and benzene.  An example is also given in which the treatment is carried out by washing the complex, drying and comminuting to a fine powder and heating with an excess of ethyl-2-hexoic acid to yield a liquid phase and a solid phase containing the mannitol which is recovered by filtration or centrifugation.